Standard Foil Research Articles

Study on the Hydrodynamic Performance of Swing-Type Flapping Hydrofoil Bionic Pumps Affected by Foil Camber

The flapping hydrofoil bionic pump is an innovative hydrodynamic device that utilizes flapping hydrofoil technology. Flapping hydrofoil bionic pumps are crucial in addressing issues like inadequate river hydropower and limited water purification capabilities in flat river network regions. Optimizing the foil characteristics is essential for enhancing the hydrodynamic efficiency of the flapping hydrofoil bionic pump. This study investigates the impact of foil camber parameters on the hydrodynamic performance of swing-type asymmetric flapping bionic pumps. The NACA series standard foils with varying cambers are analyzed using the overlapping grid technology and finite volume method. The thrust coefficient, flow rate, pumping efficiency, and flow field structure of the flapping hydrofoil bionic pump are examined under pressure inlet conditions with the foil camber. The findings indicate that increasing the foil’s curvature within a specific range can greatly enhance the maximum values of thrust coefficient, propulsive efficiency, and pumping efficiency of the flapping hydrofoil bionic pump. Specifically, when the foil curvature is 6%c, the maximum value of the instantaneous thrust coefficient of the flapping hydrofoil bionic pump is significantly improved by 31.25% compared to the symmetric foil type under the condition of an oscillating frequency of f = 1 HZ. The flapping hydrofoil bionic pump achieves its maximum pumping efficiency when the oscillation frequency is within the range of f ≤ 2.5 Hz. This efficiency is 11.7% greater than that of the symmetric foil, and it occurs when the foil curvature is 8%c. Within the frequency range of f > 2.5 Hz, the flapping hydrofoil bionic pump that has a foil curvature of 6%c exhibits the highest enhancement in pumping efficiency. It achieves a maximum increase of 12.8% compared to the symmetric foil type. Nevertheless, the average head was less than 0.4 m, making it suitable for ultra-low-head applications.

Novel Current Collector for High Energy Density Lithium-Metal Batteries

The energy density of lithium-metal batteries (LMBs) relies to a substantial fraction on the thickness of the lithium-metal anode.1,2 Additionally, the commonly used copper current collector adds a significant mass to the electrode weight (e.g., 77% when considering a 50 μm thick lithium and 10 μm thick copper), but the thinner the lithium foil, the more important becomes the current collector to provide suitable mechanical properties and ensure a good electronic contact across the whole negative electrode.Herein, we report the design of a novel current collector (patent application pending) that enables a substantial reduction in mass, while providing high electronic conductivity and suitable mechanical properties. Moreover, LMBs with a thin lithium foil and a Ni-rich NMC cathode comprising such new current collector reveal a superior performance compared to those containing a standard copper foil. References 1 Liu, J. et al. Pathways for practical high-energy long-cycling lithium metal batteries. Nat. Energy 4, 180-186, (2019).2 Niu, C. et al. Balancing interfacial reactions to achieve long cycle life in high-energy lithium metal batteries. Nat. Energy, 6, 723–732, (2021).

Investigation of the corrosion behavior influence of carbon interlayer on aluminum collector in lithium-sulfur battery system

Due to the shuttle effect brought by polysulfides, the commercialization of lithium-sulfur batteries still faces significant difficulties. Carbon interlayer is introduced to enhance the electrochemical performance of lithium-sulfur batteries, while the drawback that the interlayer will reversely affect the performance of the current collector was ignored. The current collector has a significant influence and contribution to the cycle stability of lithium-sulfur batteries. In this study, we looked into the electrochemical stability and corrosion situation of standard aluminum foil collectors in lithium-sulfur batteries with carbon interlayer. The results show that the introduction of carbon interlayer into the lithium-sulfur batteries will make aluminum foil and carbon paper form an electronic conduction path and galvanic corrosion occurs, accelerating the corrosion of aluminum foil. Therefore, we propose a new corrosion model. During the cycle, the surface of the aluminum collector becomes a non-dense passivation film and lithium polysulfide terminal (ST−1). Then, under the action of the carbon interlayer, the passivation film peels off and the oxide film is attacked by the electrolyte, thiosulfate and polythionate, resulting in pitting holes and ultimately accelerating the corrosion of the aluminum foil collector. This research has some guiding significance for the commercial application of lithium-sulfur batteries.

(Invited) Stable Anode-Free All-Solid-State Lithium Battery through Tuned Metal Wetting on the Copper Current Collector

It is demonstrated that a standard copper foil current collector with appropriately modified surface chemistry will allow for stable lithium cycling in an anode-free all-solid-state lithium battery (AF-ASSB). An argyrodite solid electrolyte Li6PS5Cl (LPSCl) is employed without any modifications or secondary interlayers. The tuned lithophilic copper surface allows for stable plating and stripping, reducing the overpotentials and significantly improving the Coulombic efficiency (CE). During continuous plating experiments using half-cells (1 mA cm-2), the accumulated thickness of electrodeposited Li on Li2Te-Cu is more than 70 μm, which is the thickness of Li foil counter-electrode. Full AF-ASSB with NMC811 cathode delivers an initial CE of 83% at 0.2C, with a cycling CE above 99%. Post-mortem cryo-FIB-SEM analysis demonstrates uniform electrodeposited metal microstructure, with no signs of voids or dendrites at the collector-SE interface. The same copper current collector but unmodified leads to inhomogeneous Li plating, with “dead metal” formation and an irregular honeycomb-like solid electrolyte interphase (SEI) interspersed with pores. Density functional theory and mesoscale calculations provide complementary insight regarding nucleation-growth behavior. Unlike for conventional liquid-electrolyte metal batteries, the role of current collector/support lithiophilicity has not been explored for emerging AF-ASSBs. Figure 1

Influence of sustainable packaging material and packaging conditions on physicochemical, microbiological, and sensorial properties of cheeses

The aim of this study was to evaluate the influence of different packaging materials [standard foil: BOPP (biaxially oriented polypropylene)/PET (polyester)/PE (polyethylene) for upper layer, and APET (polyethylene terephthalate)/PE for bottom layer; foil 1: PP (polypropylene)/PET/PE/EVOH (ethylene-vinyl alcohol copolymer)/PE upper layer, and PP/PE/EVOH/PE bottom layer; foil 2: PP/PET/PE/EVOH/PE upper layer, and PA (polyamide)/EVOH/PE bottom layer; foil 3: PP/PET/PE upper layer, and PA/EVOH/PE bottom layer; foil 4: PP/PET/PE upper layer, and PA/PE bottom layer; foil 5: PP upper layer, and PP/PP bottom layer] on the quality of 3 different ripening rennet cheeses packed under different modified atmosphere (MAP) conditions as reflected in particular physicochemical, microbiological, and sensorial changes. The changes were monitored during a period of 90 d of storage at 2°C or 8°C. For Gouda cheese, CO2 content of the headspace of the packages was in the range 35% to 45%, whereas for Maasdamer and Sielski Klasyczny cheeses it was 55% to 65%. Three-way ANOVA showed that the foil type influenced the moisture content of Gouda cheese stored for 90 d at 2°C and for Sielski Klasyczny cheese at 8°C, whereas the moisture content was not dependent on MAP conditions during storage. Moreover, the foil type had a significant effect on free fatty acid changes for Gouda and Sielski Klasyczny cheeses stored at 2°C for 90 d. Sensory attributes changed significantly over storage time at 2°C for all studied cheeses as affected by foil type, whereas there was no effect of MAP conditions. In general, the cheeses packed in standard foil and foil 4 were characterized by the highest values of mean sensory attributes. Time was the most significant factor influencing most changes in physicochemical and sensory attributes of cheeses stored at 2°C and 8°C. The storage temperature did not affect the moisture of the samples during storage. In general, we found an increase in the pH value during storage regardless of storage temperature. It was possible to decrease the thickness of the packaging material from initial 103 and 250 µm (standard foil; lid and bottom, respectively) to 98 and 100 µm (foil 4) without affecting sensory attributes of the product.

Study of the response characteristics of an online electrochemical mass spectrometry system for gas analysis of lithium-ion cells using chronoamperometry

Online electrochemical mass spectrometry (OEMS) is a promising analytical technique to monitor minor side reactions with gaseous species, taking place while charging and discharging a lithium-ion cell. However, besides the manifold examples of these custom-made systems and their application, a clear analytical view on the origin of the evolving gasses and their manifold interactions within the cell environment is missing and therefore given in this work. To get a better understanding of the complexity of gas evolution associated with electrochemical reactions in lithium-ion cells, the use of chronoamperometry as an analytical method was chosen. This led to a precise variation of the applied voltage and voltage-pulse length and enabled therewith a clear starting point of the electrochemically triggered reactions. It was found that chronoamperometry can be used to precisely trigger those reactions with gaseous products. Additionally, it was found that the release of gaseous species depends on many parameters including the cell configuration, the current, and the gas species. The response time determination showed that a custom-made highly porous electrode configuration had an overall better response behavior within minutes with differences for the respective gasses of interest, compared to a standard foil configuration cells. The herein presented methodology shows how an electroanalytical approach can help gain further insight into advanced hyphenated methods, such as OEMS in the context of studies of lithium-ion cells.Graphical abstract

Towards the use of SDD as an absolute detector for high-energy neutron measurements

As of today, the standard method employed in tokamaks for the absolute measurement of the neutron flux (thus of the nuclear fusion power) is based on activation foils, being the most robust and unbiased technique for the absolute determination of neutron fluence. However, this technique is not able to provide real-time data useful for the control of future fusion plants like DEMO. In this paper, we present some preliminary results about the R&D activity aimed at developing the Single-crystal Diamond Detectors (SDD) used for fast neutron measurements into an absolute neutron flux monitor. Tests have been conducted at the new NILE neutron source of the Rutherford-Appleton Laboratory, a facility with compact neutron generators with a maximum yield of 109 n/s and 1010 n/s for 2.5 MeV and 14 MeV neutrons, respectively. A series of neutron spectra and flux measurements have been taken with different SDD and associated DAQ. Comparisons with standard activation foils (and namely Fe, Zr, Al and Nb foils for 14 MeV neutrons and In for 2.5 MeV neutrons) and with other reference detectors are presented and discussed. Also discussed is the stability of the SDD over time when employed at high neutron rates in realistic neutron environment, and the effects of neutron irradiation on both the counting rate and detector resolution.

Hybrid Foil-Litz Windings for Highly Efficient and Compact Medium-Frequency Transformers

Medium-frequency transformers (MFTs) are key components in solid-state transformers (SSTs), where they provide galvanic isolation and a certain voltage transfer ratio between a medium-voltage (MV) grid and a low-voltage (LV) bus. Typically, MFTs are operated in the kilohertz range, which results in a significantly reduced volume and material usage compared to 50/60 Hz transformers. Foil conductor is an attractive solution for improving the filling factor and the cost-effectiveness of MFTs, however, the insulation clearances increase the ohmic losses due to the current crowding effect; besides, the electric field hotspots at the winding corners require shielding by equipotential rings. In this work, a new hybrid topology is presented, where the first and last turn of the foil winding package is replaced by turns of litz wire connected in series, one above and one below the foil winding, respectively, such that they act as equipotential shielding rings; besides, the current in the litz turns deflects the magnetic field lines away from the foil edges, decreasing the losses due to current crowding. The effect of the litz rings on the losses is modeled and measured on a hybrid and a standard foil winding prototype; a loss decrease higher than 15% is observed. The verified model is used to compare the conductor and footprint saving between windings designed with litz rings, and with standard equipotential shielding rings; in this respect, a minor amount of litz wire allows reducing losses up to 30%, whereas the conductor utilization and winding height can be decreased more than 20%, improving the power density of the MFT.

Geogrid stabilisation application – review of strain measurement methods, research and implications on design approaches

Geogrids have been used in the construction industry for stabilisation applications for decades, however, there is still a lot of conflicting information relating to the mechanical properties required to achieve this stabilisation function. Many designers adopt traditional tensile strength against tensile strain approaches that reflect membrane effect and ignore the actual mechanism of interlock and confinement. Recent ISO 10318-1:2015 defines geogrid stabilisation function by means of aggregate interlock and confinement, whereas tensile strength is seen as a function of reinforcement. Researchers across the globe have attempted to quantify the strain developed in geogrid used in a stabilising function exposed to heavy loads. Instruments used for monitoring typically comprise of standard wire strains gauges, fibre optics or foil strain gauges. Considering the harsh construction environment where the geogrids, and hence the strain gauges are exposed to damage or penetration by sharp aggregate edges, installation of sophisticated and sensitive monitoring systems can be challenging. There are several issues that can affect monitoring results such as the size of the monitoring instrument or the ability to achieve an adequate bond to the grid. Monitoring results have a major influence on geotechnical design and how practitioners perceive stabilisation mechanism in geogrids. Whilst the mechanisms of interlock and increased confinement have been studied, there still exists few design tools to provide designers with a straightforward analysis. This paper reviews research on the methods for in-situ strain measurement of geogrid installed in a stabilisation function, summarises results obtained by researchers and provides a summary of how these are reflected in geotechnical design approaches for geogrid stabilised soil.

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Target manufacturing by Spark Plasma Sintering for efficient 89Zr production

Zirconium-89 (89Zr) is an emerging radionuclide for positron emission tomography (PET), with nuclear properties suitable for imaging slow biological processes in cellular targets. The 89Y(p,n)89Zr nuclear reaction is commonly exploited as the main production route with medical cyclotrons accelerating low-energy (< 20 MeV) and low-current (< 100 μA) proton beams. Usually, natural yttrium solid targets manufactured by different methods, including yttrium electrodeposition, yttrium sputtering, compressed yttrium powders, and foils, were employed. In this study, the Spark Plasma Sintering (SPS) technique has been investigated, for the first time, to manufacture yttrium solid targets for an efficient 89Zr radionuclide yield. The natural yttrium disc was bonded to a niobium backing plate using a commercial SPS apparatus and a prototype machine assembled at the University of Pavia. The resulting targets were irradiated in a TR19 cyclotron with a 12 MeV proton beam at 50 μA. A dedicated dissolution module, obtained from a commercial system, was used to develop an automated process for the purification and recovery of the produced 89Zr radionuclide. The production yield and recovery efficiency were measured and compared to 89Zr produced by irradiating standard yttrium foils. SPS manufactured targets withstand an average heat power density of approximately 650 W∙cm−2 for continuous irradiation up to 5 h without visible damage. A saturation yield of 14.12 ± 0.38 MBq/μAh was measured. The results showed that the obtained 89Zr production yield and quality were comparable to similar data obtained using standard yttrium foil targets. In conclusion, the present work demonstrates that the SPS technique might be a suitable technical manufacturing solution aimed at high-yield 89Zr radioisotope production.

Measurement of production cross sections of 66,67Ga, 65Zn and 57,58Co from alpha induced reactions of natural copper for energy up to 50 MeV utilizing cyclotron facility at VECC

Excitation functions of 66,67Ga and 65Zn, generally used for beam intensity monitoring and/or beam energy determination; 57Co and 58Co produced by natCu(α,x) reactions were measured from their respective thresholds up to 50 MeV using standard stacked foil activation technique followed by high resolution gamma ray spectrometric method. Thick target yields were calculated for these radionuclides using the experimentally determined cross sections and stopping power of natural Cu. Experimentally obtained results were compared with the recently published data, IAEA recommended data and TENDL-2019 data based on the TALYS-1.95 code. The present data are found to be in good agreement with the literature data within experimental uncertainties.

High-accuracy mass attenuation coefficients and X-ray absorption spectroscopy of zinc - the first X-ray Extended Range Technique-like experiment in Australia.

The first X-ray Extended Range Technique (XERT)-like experiment at the Australian Synchrotron, Australia, is presented. In this experiment X-ray mass attenuation coefficients are measured across an energy range including the zinc K-absorption edge and X-ray absorption fine structure (XAFS). These high-accuracy measurements are recorded at 496 energies from 8.51 keV to 11.59 keV. The XERT protocol dictates that systematic errors due to dark current nonlinearities, correction for blank measurements, full-foil mapping to characterize the absolute value of attenuation, scattering, harmonics and roughness are measured over an extended range of experimental parameter space. This results in data for better analysis, culminating in measurement of mass attenuation coefficients across the zinc K-edge to 0.023-0.036% accuracy. Dark current corrections are energy- and structure-dependent and the magnitude of correction reached 57% for thicker samples but was still large and significant for thin samples. Blank measurements scaled thin foil attenuation coefficients by 60-500%; and up to 90% even for thicker foils. Full-foil mapping and characterization corrected discrepancies between foils of up to 20%, rendering the possibility of absolute measurements of attenuation. Fluorescence scattering was also a major correction. Harmonics, roughness and bandwidth were explored. The energy was calibrated using standard reference foils. These results represent the most extensive and accurate measurements of zinc which enable investigations of discrepancies between current theory and experiments. This work was almost fully automated from this first experiment at the Australian Synchrotron, greatly increasing the possibility for large-scale studies using XERT.

Mass spectrometric study of the leak tightness of products made of technical sintered beryllium

Beryllium products exhibiting a low level of absorption of the radiation energy are widely used in scientific instrumentation design (x-ray technology, radiation detectors, etc.). We present the results of studying the leak tightness of products (disks, plates) made of technical sintered beryllium of standard purity and foil obtained by «warm» rolling from high-purity beryllium. The relevant standards and requirements for testing are given. The leak tightness control was performed using mass spectrometric helium leak detectors with forevacuum backing pumps (oil and dry diaphragm pumps) and specialized vacuum equipment. The parameters of tightness of samples made of technical sintered beryllium were determined. The level of the helium signal during blowing was (0.6 – 7.4) × 10–11 Pa · m3/sec, which corresponds to the tightness standard of foreign analogues and matches the requirements of domestic manufacturers of x-ray equipment. The data spread tended to increase due to the growth of the background value. The obtained results can be used to improve high-tech equipment intended for flaw detection, medical devices, rapid analysis of ore raw materials, radiation safety equipment, etc.

Creep adjustment of strain gauges based on granular NiCr-carbon thin films

Abstract. An important property of high-precision mechanical sensors such as force transducers or torque sensors is the so-called creep error. It is defined as the signal deviation over time at a constant load. Since this signal deviation results in a reduced accuracy of the sensor, it is beneficial to minimize the creep error. Many of these sensors consist of a metallic spring element and strain gauges. In order to realize a sensor with a creep error of almost zero, it is necessary to compensate for the creep behavior of the metallic spring element. This can be achieved by creep adjustment of the used strain gauges. Unlike standard metal foil strain gauges with a gauge factor of 2, a type of strain gauges based on sputter-deposited NiCr-carbon thin films on polymer substrates offers the advantage of an improved gauge factor of about 10. However, for this type of strain gauge, creep adjustment by customary methods is not possible. In order to remedy this disadvantage, a thorough creep analysis is carried out. Five major influences on the creep error of force transducers equipped with NiCr-carbon thin-film strain gauges are examined, namely, the material creep of the metallic spring element (1), the creep (relaxation) of the polymer substrate (2), the composition of the thin film (3), the strain transfer to the thin film (4), and the kind of strain field on the surface of the transducer (5). Consequently, we present two applicable methods for creep adjustment of NiCr-carbon thin- film strain gauges. The first method addresses the intrinsic creep behavior of the thin film by a modification of the film composition. With increasing Cr content (at the expense of Ni, the intrinsic negative creep error can be shifted towards zero. The second method is not based on the thin film itself but rather on a modification of the strain transfer from the polyimide carrier to the thin film. This is achieved by controlled cutting of well-defined deep trenches into the polymer substrate via a picosecond laser.

Laser-driven proton acceleration from ultrathin foils with nanoholes

Structured solid targets are widely investigated to increase the energy absorption of high-power laser pulses so as to achieve efficient ion acceleration. Here we report the first experimental study of the maximum energy of proton beams accelerated from sub-micrometric foils perforated with holes of nanometric size. By showing the lack of energy enhancement in comparison to standard flat foils, our results suggest that the high contrast routinely achieved with a double plasma mirror does not prevent damaging of the nanostructures prior to the main interaction. Particle-in-cell simulations support that even a short scale length plasma, formed in the last hundreds of femtoseconds before the peak of an ultrashort laser pulse, fills the holes and hinders enhanced electron heating. Our findings reinforce the need for improved laser contrast, as well as for accurate control and diagnostics of on-target plasma formation.

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Summary Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation.

Geometrical effects of microchannel plates: Grazing incidence operation of time-of-flight mass spectrometry and comparison to standard carbon foil.

Direct detection of plasma composition in situ is integral to our understanding of ionospheric, magnetospheric, and heliospheric dynamics. Heavy, singly charged ions have been shown to contribute significantly to the mass and energy redistribution within the magnetosphere. However, most studies neglect the individual contributions of CNO group ions (instead attributing all effects to oxygen) due to the lack of mass separation of the spectrometer. In this paper, we investigate how the mass resolution of a traditional Time-of-Flight (ToF) mass spectrometer responds to different geometric applications of "grazing incidence" scattering. We apply this technique by utilizing standard Microchannel Plates (MCPs) to generate the "Start" signal, instead of the traditional carbon foil, reducing overall scattering via physically limiting plasma collisions to shallow angles of incidence. We considered only geometric differences, comparing LD 20 and LD 40 (length to diameter ratio) MCP effects on mass resolution observed by the ion composition and distribution function analyzer; traditional carbon foil spectra are contrasted against that produced by replacing one foil with these MCPs and subsequently illuminated with H+, H2 +, He+, N+, H2O+, N2 +, Ar+ ions over energies from ∼15 keV to 29 keV, similar to those seen in magnetospheric plasma. We found that among the MCP geometries, the increasing LD ratio corresponds to spectra that observe smaller FWHM and faster mean ToF, indicative of a significant drop in scattering. Additionally, the mass resolution is significantly better than when using the traditional carbon foil for masses above 4 amu. However, the geometry limits detection efficiencies due to the plasma's reduced cross section of collision with the surface that generates the "Start" signal.

Novel Cathode Electrode with Superior Rate Capability Using Perforated Aluminum Current Collector for Lithium-Ion Batteries

Electrodes were fabricated using a perforated aluminum current collector and a standard aluminum foil, and the relationship between the electrochemical performance of the battery and the current collector was investigated. The perforated aluminum foil improved the contact characteristics between the cathode materials particles and the current collector. Also, electrochemical performance indicators such discharge capacity and rate characteristics were improved due to the increased adhesion of the electrode using the perforated current collector.

Measurements of the charging-up effect in Gas Electron Multipliers

Gas Electron Multipliers (GEM) are widely used as amplification stages in gaseous detectors exposed to high rates. The GEM consists of a polyimide foil which is coated by two thin copper layers. Charges collected into the holes or created during the amplification process may adhere to the polyimide part inside the holes. This is often accompanied by a change of the effective gain. The effect is commonly known as the “charging-up effect”.This work presents a systematic investigation of the effect under well-controlled and monitored conditions for a single standard GEM foil in Ar/CO2 (90/10) gas for varying rates of X-ray interactions in the detector and for different gains of the foil. In order to cover a wide range of different rates, we apply two different methods. The first one is based on a current measurement while the second one relies on the analysis of 55Fe spectra over time. Both methods give consistent results, showing an initial increase of the effective gain with time and an asymptotic saturation, which can be well described by a single-exponential function. We find that the characteristic time constants extracted from our measurements scale inversely proportional to the rate of incoming electrons for a given GEM voltage. Introducing characteristic quantities, which describe either the number of incoming electrons per hole or the total number of electrons in a hole per characteristic time, we find consistent numbers for measurements taken at the same GEM voltages. For measurements taken at different GEM voltages, however, also the characteristic total number of electrons inside the hole, which is supposed to take into account the different effective gains, is found to be higher by a factor of about 3.5 for UGEM=350V (ntot=8.8×108) compared to 400V (ntot=2.4×108). This hints at a residual dependence of the charging-up characteristics on the GEM voltage.